The present invention relates generally to methods and devices for optical disk recording based on a mark-length recording scheme, which irradiate a laser light beam onto a recording surface of an optical disk to form pits and lands therein representing desired information. More particularly, the present invention relates to an optical disk recording device and method which achieve improved quality or characteristics of recording signals, such as reduced jitter and deviation and a lower error Irate, when information is recorded at a higher speed than a normal (or one-time) speed or when information is recorded at high density.
The CD-WO (Compact Disk-Write Once) standard, sometimes called an xe2x80x9corange book standardxe2x80x9d, is among various known schemes for recording on writable optical disks. According to the CD-WO standard, desired information is recorded in combinations of pits and lands (i.e., portions between the pits) using a xe2x80x9c3T-11Txe2x80x9d format: xe2x80x9c1Txe2x80x9d represents a time length of 231.3 ns. (={fraction (1/4.3218)} MHz) in a normal-speed (one-time-speed) recording mode, xc2xd of the normal-speed time length in a double-speed recording mode, xc2xc of the normal-speed time length in a quadruple-speed recording mode, ⅙ of the normal-speed time length in a six-times-speed recording mode. As shown in FIG. 2, a laser light beam to be used for recording on a CD-WO (or CD-R) disk is set to a top or recording power level or value, i.e., a high level capable of recording information, for each pit-forming section and set to a bottom or reproducing power value, i,e, a low capable of reproducing information but incapable of recording information, for each land-forming section. If, in this case, the laser light beam continues to be held at the top power level over a time period corresponding exactly to a desired length of a pit, the actual length of the formed pit undesirably tends to be longer than the desired length by about 1T due to residual heat of the laser light. To avoid this inconvenience, there has been employed a laser power modulation called a xe2x80x9c(nxe2x88x92K)T+xcex1(nT)xe2x80x9d recording strategy, in accordance with which the duration of each top power irradiation, intended for formation of a pit, is made shorter than a desired length nT of a pit to be formed by about a length of Kxc3x97T (K is a constant). Here, xe2x80x9cxcex1(nT)xe2x80x9d represents an amount of fine adjustment per pit length which is to be added to each pit-forming top power irradiation period to delay termination of the top power irradiation, and is set in accordance with the following relationship:
xcex1(3T)xe2x89xa7xcex1(4T)xe2x89xa7xcex1(5T), . . . ,xe2x89xa7xcex1(11T)
xe2x80x83(xcex1(3T) greater than xcex1(11T))
As another example of the laser power modulation, there has been proposed a xe2x80x9c(nxe2x88x92K)T+xcex1(nT)xe2x88x92xcex2(mT)xe2x80x9d recording strategy, where the duration of each top power irradiation is modified in accordance with a desired length nT of a pit to be formed and a length of a preceding land. Here, xe2x80x9cKxe2x80x9d is a constant. xe2x80x9cxcex1(nT)xe2x80x9d represents an amount of fine adjustment per pit length which is to be added to the end of each top power irradiation period to delay termination of the top power irradiation, and at least
xcex1(3T)xe2x89xa7xcex1(4T)xe2x89xa7xcex1(5T), . . . ,xe2x89xa7xcex1(8T)
(xcex1(3T) greater than xcex1(8T))
Further, xe2x80x9cxcex2(mT)xe2x80x9d represents an amount of fine adjustment for each preceding land""s length which is to be added to the beginning of each top power irradiation period to delay a start of the top power irradiation, and at least
xcex2(3T)xe2x89xa7xcex2(4T)xe2x89xa7xcex2(5T), . . . ,xe2x89xa7xcex2(8T)
(xcex2(3T) greater than xcex2(8T))
Furthermore, the assignee of the present application has proposed another form of recording power modulation in Japanese Patent Application No. HEI-8-233596, in accordance with which the top power level or value is increased by 1 mW for a 5T period at the beginning of each pit-forming top power irradiation period, as shown in FIG. 3, to thereby minimize unwanted jitter and pit deviation (i.e., deviation from predetermined or accurate pit lengths).
Time resolution (i.e., smallest time-variable amount) of the above-mentioned fine adjustment amounts xcex1(nT) and xcex2(mT) depends on an oscillation frequency of a crystal oscillator employed. For example, where a crystal oscillator of a 33.8 MHz oscillation frequency is used to generate 276-MHz clock pulses through electric circuit processing of xe2x80x9c33.8xc3x974xc3x97(98/96)xc3x972xe2x80x9d, there is achieved a time resolution of 1/276 MHz=3.6 ns. Such time resolution may be sufficient at low recording speeds; however, as the recording speed is increased, the length of 1T becomes smaller and hence the ratio of the time resolution relative to the 1T time length becomes considerably greater. Thus, in the prior art, it was not possible to set the fine adjustment amounts xcex1(nT) and xcex2(mT) such that the jitter, deviation and error rate fall within permissible ranges.
For instance, when recording is effected in the six-times-speed recording mode, the time length of 1T is 38.55 ns. (231.3 ns./6), and the 3.6 ns. time resolution amounts to 3.6/38.55=0.09 T; that is, the fine adjustment amounts xcex1(nT) and xcex2(mT) in this case can be set in steps of 0.09T. FIGS. 4 and 5 show measurements of jitter in a 3T land and deviation of a 3T pit when recording was effected with this time resolution, on an optical disk containing phthalocyanine and made by a certain manufacturer, in the six-times-speed recording mode using the laser power modulation of FIG. 3 with a basic strategy of xe2x80x9c(nxe2x88x920.2)T+(n)xe2x88x920.09Txe2x80x9d. In FIGS. 4 and 5, curve A represents a characteristic when xcex1(3T) was set to xe2x80x9c0xe2x80x9d, curve B represents a characteristic when xcex1(3T) was set to xe2x80x9c0.09Txe2x80x9d and curve C represents a characteristic when xcex1(3T) was set to xe2x80x9c0.19Txe2x80x9d. Horizontal axis xcex2(%) represents a standardized parameter for asymmetry of reproduced waveform which is different from the fine adjustment amount xcex2(mT).
It is required that the jitter of the 3T land be 35 or less in a 0-8% range of the parameter xcex2(%) and the deviation of the 3T pit be 40 or less in the 0-8% range of the parameter xcex2(%). In the example of FIG. 4, the 3T-land""s jitter condition was optimized when xcex1(3T) was xe2x80x9c0.19Txe2x80x9d (curve C); however, in the example of FIG. 5, the 3T-pit""s deviation exceeded the upper allowable limit of 40. Further, when xcex1(3T) was xe2x80x9c0.09Txe2x80x9d (curve B), the 3T-pit""s deviation fell within the permissible range but the the 3T-land""s jitter proved considerably less favorable than when xcex1(3T) was xe2x80x9c0.19Txe2x80x9d (curve C).
With the prior art, it was not possible to set the fine adjustment amounts xcex1(nT) and xcex2(mT) that optimize the jitter, deviation, error rate, etc., because the time resolution of the adjustment amounts xcex1(nT) and xcex2(mT) would become lower as the recording speed is raised, as stated above. The fine adjustment amounts xcex1(nT) and xcex2(mT) may be set to optimum values by raising the oscillation frequency of the crystal oscillator to thereby provide higher time resolution; however, raising the oscillation frequency of the crystal oscillator would undesirably lead to a higher cost of the device.
The adjustment amounts xcex1(nT) and xcex2(mT) in the aforementioned xe2x80x9c(nxe2x88x92K)T+xcex1(nT)xe2x80x9d and xe2x80x9c(nxe2x88x92K)T+xcex1(nT)xe2x88x92xcex2(mT)xe2x80x9d recording strategies are intended to achieve improved quality of recording signals, such as less jitter, by canceling errors that would be caused, at the beginning or fore end of the pits (i.e., the rear end of the lands) and at the rear end of the pits (i.e., the beginning of the lands), due to a difference in the amount of heat flowing from a preceding recorded portion. However, modulation by the adjustment amounts xcex1(nT) and xcex2(mT) alone could not provide a sufficiently improved quality of recording signals; rather, the recording signal quality would be deteriorated as the pits and lands are made smaller in length such as in high-density recording.
It is therefore an object of the present invention to provide an optical disk recording device which achieves improved quality, such as less jitter and deviation and a lower error rate, of recording signals in high-speed recording, without having to raise time resolution of fine adjustment amounts employed for laser power modulation.
It is another object of the present invention to provide an optical recording method which achieves improved quality of recording signals.
According to an aspect of the present invention, there is provided an optical disk recording device for forming pits and lands based on a mark-length recording scheme by irradiating a laser light beam onto a recording surface of an optical disk, the laser light beam being set to a top power value capable of forming a pit for each pit-forming section and set to a bottom power value incapable of forming a pit for each land-forming section between the pit-forming sections, which comprises a control unit that, in recording at a speed higher than a normal speed, performs: control to cause end timing of a top power irradiation period of the laser light beam to differ from predetermined end timing of a pit to be formed by a specific amount depending on a desired length of the pit or to cause start timing of the top power irradiation period of the laser light beam to differ from predetermined end timing of a land to be formed by a specific amount depending on a desired length of the land; and control to form and impart a top-power-increasing additional pulse to a portion of the top power irradiation period to temporarily increase a value of the top power value by an amount smaller than a difference between the top power value and the bottom power value and also to vary a time length of the additional pulse in accordance with the desired length of the pit, and/or, control to form and impart a bottom-power-off pulse to a portion of each land-forming bottom power irradiation period of the laser light beam to temporarily decrease a value of the bottom power value by an amount smaller than a difference between the top power value and the bottom power value and also to vary a time length of the bottom-power-off pulse in accordance with the desired length of the land.
Making longer the duration of the top-power-increasing additional pulse increases the energy to be applied for forming a pit, so that the pit would be formed with its rear end expanded or located farther from its leading or fore end. Conversely, making shorter the duration of the top-power-increasing additional pulse decreases the necessary energy for forming a pit, so that the pit would be formed with its rear end contracted or located closer to its fore end. If the power increase of the top-power-increasing additional pulse over the remaining portion of the top power irradiation period is smaller than the difference between the top power level and the bottom power level, the energy value of the top-power-increasing additional pulse per time resolution can be smaller than the energy value of the top power per time resolution (i.e., energy value for the fine adjustment amount xcex1(nT) per time resolution). Thus, even where the top-power-increasing additional pulse and the fine adjustment amount xcex1(nT) are of the same time resolution, the amount of energy to be applied can be adjusted more finely by controlling the duration of the top-power-increasing additional pulse rather than controlling the adjustment amount xcex1(nT), which allows the location of the pit""s rear end to be adjusted finely. As a consequence, the pit""s rear end can be formed at an accurate location by first roughly adjusting the fine adjustment amount xcex1(nT) for the overall duration of the top power irradiation period and then further adjusting the duration of the top-power-increasing additional pulse.
Making longer the duration of the bottom-power-off pulse decreases the energy to be applied for forming a land, so that the land would be formed with its rear end (i.e., the fore end of a following pit) expanded rearward, i.e., located farther from its fore end. Conversely, making shorter the duration of the bottom-power-off pulse decreases the energy to be applied for forming a land, so that the land would be formed with its rear end contracted forward, i.e., located closer to its fore end. If the power decrease of the bottom-power-off pulse over the remaining portion of the bottom power irradiation period is smaller than the difference between the top power level and the bottom power level, the energy value of the bottom-power-off pulse per time resolution can be smaller than the energy value of the bottom power per time resolution (i.e., energy value for the fine adjustment amount xcex2(mT) per time resolution). Thus, even where the bottom-power-off pulse and the fine adjustment amount xcex2(mT) are of the same time resolution, the amount of energy to be applied can be adjusted more finely by controlling the duration of the bottom-power-off pulse rather than controlling the adjustment amount xcex2(mT), which allows the location of the land""s rear end to be adjusted finely. As a consequence, the land""s rear end can be formed at an accurate location by first roughly adjusting the fine adjustment amount xcex2(mT) for the overall duration of the bottom power irradiation period and then further adjusting the duration of the bottom-power-off pulse.
By thus adjusting the duration of the top-power-increasing additional pulse and/or bottom-power-off pulse, it is possible to improve various characteristics of recording signals, such as jitter, deviation, error rate, etc. Although the present invention may be arranged to perform, in addition to the above-mentioned control, control to vary the increase of the top-power-increasing additional pulse according to the length of each pit to be formed or the decrease of the bottom-power-off pulse according to the length of each land to be formed, the increase of the top-power-increasing additional pulse can be made the same for every pit or the decrease of the bottom-power-off pulse can be made the same for every land, in which case it is no longer necessary to variably control the power increase or decrease and there remains only the need to control the duration of the top-power-increasing additional pulse or bottom-power-off pulse. This alternative arrangement can facilitate the necessary control as a whole.
Although the top-power-increasing additional pulse may be imparted to any desired portion of the top power irradiation period, it will be more advantageous if the additional pulse is imparted to the beginning of the top power irradiation period because the necessary timing control is significantly facilitated as compared to a situation where the additional pulse is imparted to some intermediate or mid point of the irradiation period. In addition, because the intensity of the top power irradiation tends to be greater at its beginning than at its end, imparting the additional pulse to the beginning of the top power irradiation period allows the fore end of a pit to be formed at a more accurate location than imparting the additional pulse to the end of the irradiation period and also prevents the pit""s rear end from being excessively expanded in width. Similarly, although the bottom-power-off pulse may be imparted to any desired portion of the bottom power irradiation period, it will be more advantageous if the off pulse is imparted to the beginning of the bottom power irradiation period because the necessary timing control is significantly facilitated as compared to a situation where the off pulse is imparted to some intermediate point of the irradiation period. In addition, imparting the off pulse to the beginning of the bottom power irradiation period allows the rear end of a land to be formed at a more accurate location than imparting the off pulse to the end of the irradiation period, because the power intensity drops suddenly after termination of the top power irradiation. It will also be appreciated that the top-power-increasing additional pulse may be applied only for some of various pit lengths rather than for every such pit length and the bottom-power-off pulse may be applied only for some of various land lengths rather than for every such land length.
Experiment carried out by the inventor has discovered that influences of laser light heat, on a particular pit, not only from the front but also from the rear (i.e., a portion recorded after the particular pit) tend to cause positional errors in the fore and rear ends of the pit when formed by the laser light beam and lead to poor signal quality such as a less favorable jitter characteristic.
In view of the foregoing, the present invention is characterized primarily by modifying the timing to start top-power or recording-power irradiation of the laser light beam for initiating formation of a pit (i.e., xe2x80x9crecording-power irradiation start timingxe2x80x9d) in accordance with the length of the pit to be formed, or the timing to end recording-power irradiation of the laser light beam for initiating formation of a land (i.e., xe2x80x9crecording-power irradiation end timingxe2x80x9d) in accordance with the length of the land to be formed.
Namely, according an aspect of the present invention, there is provided an optical disk recording method which comprises a step of modifying recording-power irradiation start timing of a laser light beam, to start formation of a pit, according to a length of the pit, which is characterized by, when a pit of a relatively great length is to be formed, delaying the recording-power irradiation start timing as compared to when a pit of a smaller length is to be formed, as long as respective lands immediately preceding the pit of a relatively great length and the pit of a smaller length have a substantially same length. As known in the art, more laser light heat tends to accumulate in the optical disk as the length of a pit to be formed becomes greater, which would result in the pit""s fore end being undesirably expanded forward. In the present invention, however, the arrangement of delaying the recording-power irradiation start timing for formation of a relatively long pit as compared to formation of a shorter pit can cancel the tendency of heat accumulation. Thus, the present invention allows the fore end of the pit to be formed accurately at a predetermined position and can significantly reduce unwanted jitter and deviation, thereby achieving enhanced quality of recording signals.
The present invention also provides an optical disk recording method which comprises a step of modifying recording-power irradiation end timing of the laser light beam, to start formation of a land, according to a length of the land, which is characterized by, when a land of a relatively small length is to be formed, advancing the recording-power irradiation end timing as compared to when a land of a greater length is to be formed, as long as respective pits immediately preceding the land of a relatively great length and the land of a smaller length have a substantially same length. When a land of a relatively small length is formed, the heat of the laser light beam used for forming a next pit tends to transfer via the land to a preceding pit, which would result in the preceding pit""s rear end being undesirably expanded rearward away from its fore end. In the present invention, however, the arrangement of advancing the recording-power irradiation end timing for formation of a relatively short land as compared to formation of a longer land can cancel the tendency of forward heat transfer. Thus, the present invention allows the rear end of the land to be formed accurately at a predetermined position and can significantly reduce unwanted jitter and deviation, thereby achieving enhanced quality of recording signals.
Generally, of all pits or lands of 3T-11T lengths recorded in a given optical disk in accordance with the CD-WO standard or the like, 4T pits or lands account for about 30%, pits or lands shorter than 4T account for about 36%, and pits or lands longer than 4T account for about 34%. Thus, in a preferred implementation of the present invention, the recording-power irradiation start timing for formation of a pit or land of the 4T length is set to predetermined reference timing with no modification, the recording-power irradiation start or end timing for formation of a pit or land shorter than the 4T length is advanced ahead of the reference timing, and the recording-power irradiation start timing for formation of a pit or land longer than the 4T length is delayed behind the reference timing. With this arrangement, necessary modification amounts for the recording-power irradiation start and end timing can be minimized to a near-zero value, so that it is possible to prevent d.c. components in reproduced signals from being increased due to the timing modification. More specifically, when a pit or land of the 3T length is to be formed, the recording-power irradiation start or end timing may be advanced ahead of the reference timing by an amount ranging from 0 to 12% of the 1T length, and when a pit or land of a length between 5T and 11T is to be formed, the recording-power irradiation start or end timing may be delayed behind the reference timing by an amount ranging from 0 to 6% of the 1T length.
According to another aspect of the present invention, there is also provided an optical disk recording method comprising: a step of modifying recording-power irradiation start timing of a laser light beam, to start formation of a pit, according to a combination of lengths of a pit to be formed and a land immediately preceding the pit; and a step of modifying recording-power irradiation end timing of the laser light beam, to start formation of a land, according to a combination of lengths of a land to be formed and a pit immediately preceding the land. The above-mentioned step of modifying recording-power irradiation start timing of the laser light beam includes a step of, when a pit of a relatively great length is to be formed, delaying the recording-power irradiation start timing as compared to when a pit of a smaller length is to be formed as long as respective lands immediately preceding the pit of a relatively great length and the pit of a smaller length have a substantially same length and also, when a land of a relatively small length is to be formed, delaying the recording-power irradiation start timing as compared to when a land of a greater length is to be formed. The above-mentioned step of modifying recording-power irradiation end timing of the laser light beam including a step of, when a land of a relatively small length is to be formed, advancing the recording-power irradiation end timing as compared to when a land of a greater length is to be formed as long as respective pits immediately preceding the land of a relatively small length and the land of a greater length have a substantially same length and also, when a pit of a relatively great length is to be formed, advancing the recording-power irradiation end timing as compared to when a pit of a smaller length is to be formed.
With the arrangement that the timing modification is performed according to a combination of lengths of a pit to be formed and a land immediately preceding the pit and a combination of lengths of a land to be formed and a pit immediately preceding the land, the pit""s fore and rear ends can be formed more accurately at their respective predetermined, and unwanted jitter and deviation can be significantly reduced, which thereby achieves enhanced quality of recording signals.
According to still another aspect of the present invention, there is also provided an optical disk recording device comprising: a storage unit that stores various modification amounts for recording-power irradiation start timing of the laser light beam, to start formation of a pit, corresponding to various combinations of lengths of a pit to be formed and a land immediately preceding the pit, and various modification amounts for recording-power irradiation end timing of the laser light beam, to start formation of a land, corresponding to various combinations of lengths of a land to be formed and a pit immediately preceding the land; and a control unit that detects, from an input recording signal, combinations of lengths of a pit and a land immediately preceding the pit and lengths of a land and a pit immediately preceding the land, reads out from the storage unit one of the modification amounts for recording-power irradiation start timing or the modification amounts for recording-power irradiation end timing which corresponds to one of the detected combinations, and modifies the recording-power irradiation start timing or the recording-power irradiation end timing relative to predetermined reference timing by an amount corresponding to the read-out modification amount.
One of the modification amounts for recording-power irradiation start timing or the modification amounts for recording-power irradiation end timing, corresponding to one of the detected combinations of pit and land lengths, is read out from the storage unit. With this arrangement, both the recording-power irradiation start timing and the recording-power irradiation end timing can be modified, during recording, without involving complicated arithmetic operations for determining a appropriate modification amounts, which thus permits appropriate high-speed recording.